1. Field of the Invention
The present invention relates to novel chiral ligands derived from 3,3xe2x80x2-substituted biaryl phosphines and phosphinites. More particularly, the present invention relates to transition metal complexes of these chiral phosphine ligands. The transition metal complexes are useful as catalysts in asymmetric reactions, such as, hydrogenation, hydride transfer, allylic alkylation, hydrosilylation, hydroboration, hydrovinylation, hydroformylation, olefin metathesis, hydrocarboxylation, isomerization, cyclopropanation, Diels-Alder reaction, Heck reaction, isomerization, Aldol reaction, Michael addition, epoxidation, kinetic resolution and [m+n] cycloaddition.
2. Description of the Prior Art
Discovery of new chiral ligands is crucial in developing highly enantioselective transition metal-catalyzed reactions. Many chiral ligands have been made for applications in asymmetric catalysis, however, relatively few of these chiral ligands are commonly used in industry for the synthesis of chiral molecules.
Several chiral ligands having a biaryl backbone are known in the prior art. These are summarized below: 
Among these ligands, BINAP (1) is one of the most frequently used chiral ligands. The axially dissymmetric, fully aromatic BINAP has demonstrated to be highly effective for many asymmetric reactions (Noyori, R. et al. Acc. Chem. Res. 1990, 23, 345, Ohkuma, T. et al. J. Am. Chem. Soc. 1998, 120, 13529). Recent results show that partially hydrogenated BINAP with a larger bite angle, H8-BINAP (2), is a better ligand for certain asymmetric reactions due to restriction of conformational flexibility (Zhang X. et al. Synlett 1994, 501). Chiral BINAPO (3) was made and it was not effective due to the conformational flexibility (Grubbs, R. et al. Tetrahedron Lett. 1977, 1879). Other axially dissymmetric ligands such as BIPHEMP (4) and MeO-BIPHEP (5) were developed and used for a number of asymmetric reactions (Schmid, R. et al. Pure and Appl. Chem. 1996, 68, 131; Schmide, R. et al. Helv. Chim. Acta, 1988, 71, 897). However, the present inventor is not aware of any examples of related 3,3xe2x80x2 substituted chiral biaryl phosphines, the subject of the present invention, being disclosed in the prior art (Broger, E. A. et al., WO 92/16536 and Broger, E. A. et al., WO93/15089). NAPHOS (6), another example of a prior art compound has been prepared (Tamao, K. et al. Tetrahedron Lett. 1977, 1389) and was found to be not effective for asymmetric hydrogenation reaction. The corresponding ligands with the N linker, BDPAB (7) and H8-BDPAB (8) have also been made and tested for asymmetric hydrogenation reactions (Zhang, F. et al. J. Am. Chem. Soc. 1998, 120, 5808).
The present invention includes a ligand represented by the formula or its enantiomer: 
wherein each X and Xxe2x80x2 is independently selected from the group consisting of: alkyl, aryl, substituted alkyl, substituted aryl, OR, SR, NR2, COOR, halide, SiR3, P(O)R2, P(O)(OR)2 and P(OR)2;
wherein each Z and Z1 is independently selected from the group consisting of: alkyl, aryl, substituted alkyl, substituted aryl, OR, SR, NR2, COOR, halide, SiR3, P(O)R2, P(O)(OR)2 and P(OR)2; or wherein Z and Z1 together form the bridging group A-B-A1;
wherein each Zxe2x80x2, Zxe2x80x3, Z1xe2x80x2 and Z1xe2x80x3 is independently selected from the group consisting of: H, alkyl, aryl, substituted alkyl, substituted aryl, OR, SR, NR2, COOR, halide, SiR3, P(O)R2, P(O)(OR)2 and P(OR)2, or wherein Zxe2x80x2 and Z together form the bridging group Axe2x80x2-B-A; Zxe2x80x2 and Z together form a fused cycloaliphatic or aromatic group; Z1 and Z1xe2x80x2 together form the bridging group A1-B1-A1xe2x80x2; and/or Z1 and Z1xe2x80x2 together form a fused cycloaliphatic or aromatic group;
wherein each A, Axe2x80x2, A1 and A1xe2x80x2 is independently selected from the group consisting of: O, CH2, NH, NR, S, CO and a bond;
wherein each B and B1 is independently selected from the group consisting of: linear, branched or cyclic alkylene of 1 to 6 carbon atoms, arylene of 6 to 12 carbon atoms, O, CH2, NH, NR, S, CO, SO2, P(O)R, P(O)OR, POR, SiR2 and a bond;
wherein each T is independently selected from the group consisting of: alkyl, substituted alkyl, aryl, substituted aryl, alkoxide, aryloxide, R, Rxe2x80x2, Rxe2x80x3, YRxe2x80x2, YRxe2x80x3, Yxe2x80x2Rxe2x80x2 and Yxe2x80x3Rxe2x80x3; or wherein two T groups together form an alkylene, arylene, alkylenediamino, arylenediamino, alkelenedioxyl or arylenedioxyl;
wherein each Txe2x80x2 is independently selected from the group consisting of: alkyl, substituted alkyl, aryl, substituted aryl, alkoxide, aryloxide, R, Rxe2x80x2, Rxe2x80x3, YRxe2x80x2, YRxe2x80x3, Yxe2x80x2Rxe2x80x2 and Yxe2x80x3Rxe2x80x3; or wherein two Txe2x80x2 groups together form an alkylene, arylene, alkylenediamino, arylenediamino, alkelenedioxyl or arylenedioxyl;
wherein each R, Rxe2x80x2 and Rxe2x80x3 is independently selected from the group consisting of: alkyl, substituted alkyl, aryl, aralkyl and alkaryl of 1 to 22 carbon atoms; or wherein two R groups, two Rxe2x80x2 groups or two Rxe2x80x3 group together form an alkylene or arelene group; and
wherein each Y, Yxe2x80x2 and Yxe2x80x3 is independently selected from the group consisting of: O, CH2, NH, S and a bond between carbon and phosphorus; with the proviso that when the Y group at the 2xe2x80x2 position is a bond between carbon and phosphorus, Xxe2x80x2 is hydrogen.
The present invention further includes a catalyst prepared by a process, which includes: contacting a transition metal salt, or a complex thereof, and a ligand according to the present invention.
The present invention still further includes a process for preparation of an asymmetric compound including: contacting a substrate capable of forming an asymmetric product by an asymmetric reaction and a catalyst prepared by a process including: contacting a transition metal salt, or a complex thereof, and a ligand according to the present invention.
The metal complexes are useful as catalysts in asymmetric reactions, such as, hydrogenation, hydride transfer, allylic alkylation, hydrosilylation, hydroboration, hydrovinylation, hydroformylation, olefin metathesis, hydrocarboxylation, isomerization, cyclopropanation, Diels-Alder reaction, Heck reaction, isomerization, Aldol reaction, Michael addition, epoxidation, kinetic resolution and [m+n] cycloaddition. The metal complexes are particularly effective in Ru-catalyzed asymmetric hydrogenation of beta-ketoesters to beta-hydroxyesters and Ru-catalyzed asymmetric hydrogenation of enamides to beta amino acids.